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I went to a call with a shorted comp one day. I must have been in a mood because I actually reset the breaker a few times to watch the amp draw as it snapped the breaker silly.

On the 3rd try it tripped. So I then took apart the unit to do a meg ohm test, etc. Comp shorted directly to ground. Then I noticed no more power to anything when I saw the contactor not closed anymore. The whole house was dead.

Went to the main panel expecting the main 200 AMP to be tripped (which has happened before) and nope, it wasn't.

Shortly comes a APS service truck out to the neighborhood. He opened the transformer box and I blew a 15 amp fuse on the primary side of the transformer (12,000 volts)!

Why does the Main CB trip (minimum 100A, probably 200) but not the 50 for the A/H?? Was the Question.

I was trying to answer why the 50 amp over current protective device didn't trip or fault but the 200 amp main in the house panel or main disconnect did. You have to look at available fault current, length and size of the branch circuit and feeder, and the transformer suppling the house. When you get a large spike on the system as occurred in the pictures, the circuit breaker (CB) will sometimes not trip at the first overload but may fault the first over current device on the circuit (House Main Over-current Protective Device). A circuit breaker will only fault/trip if the fault travels back to the utility transformer from the fault location and then back to the breaker. That is why molded case circuit breakers will not trip if the circuit is in an "open neutral" condition.

Originally posted by durango A circuit breaker will only fault/trip if the fault travels back to the utility transformer from the fault location and then back to the breaker. That is why molded case circuit breakers will not trip if the circuit is in an "open neutral" condition.

Maybe you should reconsider these statements.

Consider the express purpose of a ground conductor and the ground electrode conductor.

How you doing Wolfman, no I think I will stand by my statement. A grounded conductor is a neutral conductor that is bonded or intentionally connected at the main grounding point of the service equipment or separately derived system (transformer, ups, etc.)where a grounding connection is made to a grounding electrode system(foundation steel, made electrode, metal water pipe or other effectively grounded system). The grounding conductor is used solely to connect equipment or the grounded (neutral) circuit of a wiring sytem to a grounding electrode system usually by a grounding electrode conductor. This conductor connects the grounding electrode to the equipment grounding conductor , or the grounded (neutral) conductor or both at the circuit at the service equipment or at the source of a separately derived system at least according to Article 100 and 250 of the NEC 1999 Edition, sorry didn't have my 2002 available.

A molded case circuit breaker will only function if the fault travels along a grounding path back to the transformer and returns to the device. At least this is what I have been taught in continuing ed. courses. A better source for information on this might be a manufacturer than me.

A breaker or fuse is in place to protect the conductors, it is a stopping place for overcurrent and/or ground fault. The purpose is to clear the overload or fault at that point and not let it move further up the system. The purpose of the ground conductor, be it conduit, a seperate conductor, or the bare wire in NM cable; to provide a low impedance path back to the panel and hense the breaker. In the event of a ground fault the current is carried back to the panel, thru the ground, to the breaker/fuse and the fault is cleared at the first overcurrent device before the problem travels back up the system further.

If the event of an "open neutral" affecting a single pole breaker, the conductor for the unbalanced current is lost. Now the single phase breaker is going to "see" somewhere between 0 -230 volts depending on the internal resistance of devices on the other buss. This means that a 120 volt rated breaker may be seeing 50 volts or 230 volts. This accounts for the unstable trip charateristics in an "open neutral" situation. If you have ever encountered this problem, you will notice that the voltage on L1 and L2 will shift as the loads change.

BBQ

Post a pic of a pre-BBQ disconnect, same type...

I will try to find one. She has another one of the same on her Condenser.

As for the tripped Main the distance to the pair of 200A mains is about 60 feet, as the wire feeds. But I think the main only tripped on reset of the 50A air handler breaker. I hit it once before I went up to the attic and it took the whole panel down. I believe the wire was
8-3wG.
I think she did too, she was scared and said she had seen sparks from the breaker. I don't think she, still to this day, comprehends how lucky she is.

I agree with some of your statements OHM's Law will dictate the available voltage on systems which have lost their grounded conductor based on available volt-ampere per leg. Most lighting and appliance boards have 240 volt devices which will allow 120 volt loads to continue to work (althought not for long if the voltage exceeds the maximum voltage tested on the equiipment, usually 135 volts max.)
I still find in the electrical trade a lot of misunderstanding of the terms grounding, grounded, bonding, and grounding electrode systems and how the conductors are used.
I did find some lovely electrical engineering ladies at http://www.ferrazshawmut.com/contact/app_ass_na.html that offered some great insight as to how over current protective devices operate and also some good verbal information at Schneider Electric (Square D) on some the statements I have made and now feel quite comfortable in standing behide. Hope you have a good one.

POSSIBLE CONSUMER RECALL

Hello answer:

I personally think you should send these pictures along with a picture of a non burnt disconnect with model, serial, manufacture etc. to the proper authorities for a possible consumer recall if necessary being you stated all the lugs had been checked for tightness in your previous P/M.

There is no telling how many lives across this country might possibly be in danger with these. I'd also be willing to bet they don't have MADE IN USA on them either.

Originally posted by durango Hello Wolfdog.......
I still find in the electrical trade a lot of misunderstanding of the terms grounding, grounded, bonding, and grounding electrode systems and how the conductors are used.....

Here is the un-burnt disconnect.

A sguare D if you would belive it. She has two more at her house I told her to replace them, they may be a bad batch from the factory. But no response yet. You bet ya I got photos and her signature on a denial of the disconects.